4-Oxo-8-[4-(4-phenylbutoxy)benzoylamino]-2-(tetrazol-5-yl)-4H-1-benzopyran is an antagonist toward leukotrienes (LTC4, LTD4, LTE4) and is known to be a medicine having excellent pharmacological activities on various allergic diseases including asthma (JP-A 61-50977). Its 1/2 hydrate is called as "pranlukast" and is utilized as an anti-allergic medicine. Then, in the field of ophthalmology, it has been proposed to apply such a medicine to allergic eye diseases such as vernal keratoconjunctivitis, etc.
In general, for preparing an aqueous liquid pharmaceutical composition such as water-soluble eye drops, it is considered that a pharmacologically active component should be present in concentration of about 0.01 to 0.1%. However, 4-oxo-8-[4-(4-phenylbutoxy)benzoylamino]-2-(tetrazol-5-yl)-4H -1-benzopyran and its hydrate (hereinafter, they are referred to as "pranlukast" all together, unless otherwise stated) have very low water-solubility, which makes every difficult to prepare a useful aqueous liquid pharmaceutical composition thereof. Then, the use of polyvinyl pyrrolidone and .beta.-cyclodextrin has been proposed to solubilize pranlukast (JP-A 8-73353).
However, even if polyvinyl pyrrolidone, which is a material known to have strongest solubilization power, is used, pranlukast dissolves in water in concentration of, at highest, about 0.01%.